1. Field of the Invention
The present invention relates to the field of computer systems. More specifically, the present invention relates to the arbitration and controlling of arbitration for access to a serial bus.
2. Background Information
Most computer systems include buses for transporting data between agents connected to the buses. Examples of buses include serial buses such as IEEE's High Performance Serial Bus (Standard P1394 of The Institute of Electrical and Electronic Engineers, Inc., draft 8.0v2 published Jul. 7, 1995, hereinafter HPSB). HPSB is also referred to as a gap-based serial bus in that it uses arbitration reset gaps to define a fairness interval, and subaction gaps to delineate between actions by the asynchronous bus agents within a fairness interval.
The fairness interval is employed to ensure that all asynchronous bus agents get their fair share of using the bus. Each asynchronous bus agent is to arbitrate for the bus only once for each fairness interval. As soon as an asynchronous bus agent wins the arbitration, it is to refrain from further arbitrating for the bus until the next arbitration reset gap is detected. In order for the gaps to be distinguishable, each gap is relatively long. The length of an arbitration reset gap is in the order of 83 to 2069 bit periods for a 100 Mbit/sec HPSB, depending on the bus topology. At 1600 Mbits/sec, the length of each arbitration reset gap increases to about 1328 to 33104 bit periods. Thus, while this approach is easy to implement, it is inefficient.
In particular, many modern day serial bus device protocols, such as SBP2.sup.1, require a sequence of control packets to be passed between the host system and the device, prior to any movement of usable data. Under the prior art arbitration scheme, the device would have to transfer the required control packets over multiple fairness intervals, resulting in significant overhead for each transfer of data. FNT .sup.1 SBP2--Serial Bus Protocol 2, working document of the X3T10 committee, a technical committee of Accredited Standards Committe.
Thus, a more efficient approach to arbitrating and controlling arbitration for access to a serial bus is desired.